Labarte Organ Tablature
From pipe-organ.wiki
The Labarte Organ Tablature is a manuscript which was in the possession of Jules Labarte. In 1864, it was lost. However, around 1990 it reappeared in the Houghton Library at Harvard University.
The manuscript is dated to the 16th century and it contains three anonymous Alternatum organ masses.
References
[Labarte-Orgeltabulatur in Repertorium Orgelmusik (page 120)]